The present invention relates generally to signal transmitting circuits, and more specifically to circuits for transmitting high frequency digital signals. The invention is particularly applicable to systems of the type in which a data transmission link, including a fiber-optic element and associated signal conversion devices, is employed to transmit signals of a higher frequency than the normal upper operating frequency limit of the data link.
Transmission of data in digital form has become increasingly important with the advent and rapidly expanding use of digital data processors and processing techniques. Recent digital processing developments have resulted in very notable increases in the speed at which data processing can be accomplished, and in the amounts of data which can be practically processed. These developments have had the effect of markedly increasing demands on particularly the high frequency signal handling capabilities of digital data transmission circuits and systems. In addition, they have greatly increased the need for simple, low cost, high data rate circuits and systems.
One of the technologies which has recently received considerable attention in connection with the foregoing requirements is that of optical signal transmission. It is well known that optical data links potentially possess the capability of operation at much increased frequency bandwidths. However, the present state of the art is such that reasonably simple, low cost components and circuits required for practical optical data transmission have relatively low effective upper frequency limits.
At present, a typical optical system suitable for general purpose data transmission applications comprises a light emitting diode energized by an electrical data signal, a fiber-optic element for transmitting the resulting optical signal, and a photodetector for receiving the optical signal and converting it to a corresponding electrical signal. Examples of such systems are illustrated and described in the following articles:
(1) Gundlach, "Fiber-optic Developments Spark Worldwide Interest," Electronics, Aug. 5, 1976 (pages 81-104)
(2) Schmid, "Fiber-optic Data Transmission: A Practical, Low-Cost Technology," Electronics, Sept. 2, 1976 (pages 94-99)
These articles, as well as Hodapp, "Optical Isolators Yield Benefits in Many Linear Circuits," Electronics, Mar. 4, 1976 (pages 105-110), indicate that the highest frequency at which practical, relatively low cost optical data transmission systems are presently operable is about 10 mHz.
An additional data rate limitation occurs in systems utilizing Manchester coded digital signals. In such a coding format, data bits are represented by the directions of transition of signal level at equally spaced bit times. If any two successive data bits are the same, there must be an intermediate transition in signal level. This requires the transmission circuit to respond to a frequency twice the data bit rate. In addition, in some cases the system must transmit a synchronizing pulse having a duration of 3/2 of a bit period.
Manchester coding permits the use of AC coupling of digital signal circuits, and thereby avoids problems associated with drift of DC signal levels. However, its requirement of a system capable of transmitting frequencies higher than the data rate detracts from the other advantages inherent in optical data transmission systems of the above-described type.
Various schemes have been devised for increasing the bit rate of systems for transmitting Manchester coded data. For example, U.S. Pat. No. 3,979,746 issued to B. R. Jarrett on Sept. 7, 1976 discloses a Manchester coded demodulator for avoiding recovery time limitations of one-shot circuits used in some decoding arrangements to mask non-significant transitions. Such schemes do not, however, increase the upper frequency limit at which the basic signal transmission circuit can be used.
The applicant has discovered a unique technique and feedback means for permitting usable high frequency digital data signals to be transmitted over signal transmission circuits whose normal upper operating frequency limit is lower than the frequency of the data signals. Accordingly, the frequency and data rate handling capabilities of a digital signal transmission circuit comprising simple relatively low-cost components can be significantly increased.